Decentralization has been and is still high on the agenda in contemporary health sector reforms. However, despite extensive literature on the topic, little is known about the processes and results of decentralization, including the relationship with the control of major public health problems caused by communicable diseases. This paper reports from a study of decentralization and control of tropical diseases in districts implementing health sector and local government reforms in Tanzania. The study was undertaken in four districts, involving interviews and discussions with key stakeholders from individual household members to the district commissioner, and a review of official health policy, planning and management documents. The study findings reveal devolution of financial, planning and managerial authority being theoretical rather than practical, as district health plans are largely directed by national and international priorities rather than by local priorities. Vertical programmes still exist, focusing narrowly on single diseases. The local mechanisms for multisectoral collaboration, as well as community participation functions, are far from optimal. Further, inappropriate and weak information systems prevent adequate local responsiveness in setting priorities. In conclusion, decentralization might have a large potential for improving health system performance, but problems of implementation pose serious challenges to releasing this potential.